Electrochemical cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries." See, for instance, U.S. Pat. Nos. 5,229,225, 5,238,758, 5,358,801, and 5,366,829. These cells offer a number of advantages over electrochemical cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features.
Non-aqueous lithium electrochemical cells typically include an anode of metallic lithium, a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents and a cathode of an electrochemically active material, typically a chalcogenide of a transition metal. During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium ions pass from the electrochemically active material through the electrolyte and are plated back onto the lithium anode.
During each discharge/charge cycle small amounts of lithium and electrolyte are consumed by chemical reactions at newly created surfaces. As lithium inherently tends to form high surface area peaks or dendrites as it is plated back onto the anode, this reactive condition is aggravated. Furthermore, the dendritic peaks continue to grow until they eventually contact the cathode which causes the cell to fail. Additional amounts of lithium do not cohesively plate onto the anode during the charge cycle and result in the formation of spongy deposits near the anode surface. As these deposits are not in electrically conductive contact with the anode, they eventually detract from the capacity of the cell.
One approach to this problem has been to replace the lithium metal anode with a carbon anode such as coke or graphite intercalated with lithium metal to form Li.sub.x C. In operation of the cell, lithium passes from the carbon through the electrolyte to the cathode where it is taken up just as in a cell with a metallic lithium anode. During recharge, the lithium is transferred back to the anode where it reintercalates into the carbon. Because no metallic lithium is present in the cell, melting of the anode cannot occur even under abuse conditions. Also, because lithium is reincorporated into the anode by intercalation rather than by plating, dendritic and spongy lithium growth cannot occur.
The use of carbon anodes however is not without problems. As Li.sub.x C is a reactive material which is difficult to handle in air, it is preferably produced in-situ in a cell. In doing so, some of the lithium and carbon are consumed in an irreversible process. This irreversible process results in an initial capacity loss for the cell which reduces the cell's overall performance. Furthermore, the cell often exhibits a progressive loss of capacity over numerous charge/discharge cycles. This progressive loss is commonly referred to as "capacity fade."
In view of the above shortcomings associated with the prior art, there is a need for solid state electrochemical devices that are capable of providing improved cycle life, capacity, and rate capability.